A handheld RF meter is an important tool used by field network technicians to test cable modem services, digital video, analog video, and VoIP signals. The RF meter, when connected to a coaxial cable of a cable network, acts as a customer's modem enabling the validation of network services available to the customer. The testing may involve monitoring ongoing traffic, injection of predetermined test messages to monitor the system's response, measuring operational parameters, such as bit error rates, message travel times and the like, or monitoring the operation of individual components of the system.
The accuracy of such meters may be affected by the presence of an external electro-magnetic (EM) field surrounding the coaxial cable of the system being analyzed. The external electro-magnetic field induces electrical currents in an external conducting sheath, also referred to as a cable shield, of the coaxial cable. The induced electrical currents, or sheath currents, may interfere with an electro-magnetic signal propagating via a signal conductor of the cable, resulting in signal distortions and noise.
Major sources of the external EM field that can be present around an RF meter are broadcast Radio and TV signals. The over-the-air broadcast signals may affect readings of the RF meter because of a frequency overlap between the broadcast and cable TV signals. For example: the cable channel 86 has a frequency bandwidth of 595.25 MHz-599.75 MHz; and the adjacent broadcast channel 35 has a frequency bandwidth of 596 MHz-602 MHz. Thus, the Audio and Video portions of these two signals are overlapped. The similar overlap affects multiple channels across the entire spectrum from 54 MHz-890 MHz.
The electro-magnetic interference (EMI) due to the radio- and TV-broadcast may not significantly affect readings of an RF meter if there is a substantial difference in signal power, i.e. when a cable signal to be measured is significantly stronger than the interfering broadcast signal. However, TV broadcast providers have been incrementally increasing power of the over-the-air transmissions to maintain quality of the signal provided to the growing customer base over an increasing area, thus increasing the power level of the EMI signal and its effect on the cable TV signal measurements.
Furthermore, the advance of the digital TV and HDTV has increased the importance of accurate RF measurements of cable TV signals, including measurements of such parameters as modulation error rate (MER) and bit error rate (BER). The sensitivity of the digital TV signal to EMI compounds the impact of the high-power TV broadcast on the cable TV monitoring. The imminent transition of the majority of TV channels to digital format adds to the need for reliable EMI suppression in devices for monitoring cable TV signals.
An object of the present invention is to provide a handheld RF meter with an improved isolation from EMI.